How Much Excess Fresh Water Was Added to
the North Atlantic in Recent Decades?

Continued Freshening of the North Atlantic
Could Slow the Conveyor in the 21st Century

Press Release / Woods Hole Oceanographic Institution
16jun2005

[More below]

Large regions of the North Atlantic Ocean have been growing fresher since the
late 1960s as melting glaciers and increased precipitation, both associated with
greenhouse warming, have enhanced continental runoff into the Arctic and
sub-Arctic seas. Over the same time period, salinity records show that large
pulses of extra sea ice and fresh water from the Arctic have flowed into the
North Atlantic. But, until now, the actual amounts and rates of fresh water
accumulation have not been explicitly known.

Topographic
map of the Nordic Seas and Subpolar Basins, with schematic circulation of
surface currents (solid curves) and deep currents (dashed curves) that
form a portion of the Atlantic Meridional Overturning Circulation (MOC).
The color of the curves depicts their approximate temperatures. Map inset
shows the boundaries of the Nordic Seas and Subpolar Basins used in the
analysis of water volume. (Illustration by Jack Cook, Woods Hole
Oceanographic Institution)

In a paper to be published June 17 in Science, Ruth Curry of the Woods
Hole Oceanographic Institution (WHOI) and Cecilie Mauritzen of the Norwegian
Meteorological Institute quantified for the first time how much additional fresh
water caused the observed salinity changes in the northern North Atlantic Ocean,
how fast it entered the Atlantic circulation, and where that fresh water was
stored. They report that patterns of fresh water accumulation over the past four
decades suggest that a freshening threshold important to the ocean circulation
and its poleward transport of heat could be reached in a century, although
future impacts of global warming and glacial melting make prediction imprecise
at this time.

Curry, a research specialist in the WHOI Physical Oceanography Department,
and Mauritzen, an oceanographer at the Norwegian Meteorological Institute,
analysed data collected in the North Atlantic Ocean between Labrador, Greenland
and northern Europe over the last 55 years to reconstruct the history of ocean
properties such as temperature, salinity and density. For successive 5-year time
periods, they estimated how much fresh water had to have been added or removed
to account for the top-to-bottom salinity changes observed from 1953 to 2002.
They mapped the distribution of fresh water storage layer-by-layer to determine
where the excess was accumulating and how it was affecting ocean density. They
then used the observed rates and distribution of freshening to estimate how long
it would take to reach thresholds that would affect the portion of ocean
circulation driven by density.

In an average year, about 5000 cubic kilometers (km) of fresh water flows
from the Arctic into the North Atlantic through passages located east and west
of Greenland. The researchers estimate that in addition to this amount, an extra
19,000 cubic km flowed into and diluted the northern seas over the 30-year time
period between 1965 and 1995. Fully half of the excess fresh water (~10,000
cubic km) entered the system in the late 1960s boosting the flow rate from 5000
to ~7000 cubic km per year-a 40% increase in fresh water inflow. For comparison,
the outflow from the Mississippi River each year is about 500 cubic km, while
Earth’s largest river, the Amazon, discharges 5000-6000 cubic km annually.

Curry says the records for the 1950s and 1960s showed salinities initially
increasing in the Atlantic’s Subpolar Basins, while changes in the Nordic Seas
were relatively small. Between 1970 and 1995, however, both areas became greatly
freshened. The team says the most striking event in their study occurred in the
early 1970s and is known as the “Great Salinity Anomaly”. During the late
1960s, a large pulse of fresh water entered the Nordic Seas through Fram Strait
and moved quickly southward in the East Greenland Current. Curry says the event
is well-named since it contributed about 10,000 cubic km of extra fresh water to
the northern ocean basins. (Previously, the Great Salinity Anomaly was believed
to have involved less than one quarter of that amount.) This fresh water was
eventually mixed downward in the Labrador and Irminger Basins and then moved
horizontally at mid-depths around the North Atlantic.

Pulses of excess fresh water and ice appear to have come from the Arctic in
the 1980s and 1990s as well. Of the estimated 19,000 cubic km added to the
northern North Atlantic between 1965 and 1995, about 80% ended up in the
Subpolar Basins, which are about twice the geographic size of the Nordic Seas.
The amount of fresh water involved would be equivalent to a layer about 3 meters
(roughly 9 feet) thick spread evenly over the total area of the Subpolar Basins,
and a layer about 1.8 meters (about 5 feet) thick over the Nordic Seas.

The Nordic Seas (located between Iceland, Greenland and Norway) and the
Labrador and Irminger Basins (east and west of southern Greenland) are places
where cold dense waters are formed, a critical component of the meridional
overturning circulation (MOC) and part of a great “ocean conveyor belt” that
carries warm surface waters from the tropics northward. At high latitudes, the
heat-bearing surface waters cool (the heat is released to the overlying
atmosphere) and these denser waters sink and flow southward in the deep ocean-a
process which helps keep the conveyor moving. The transport of heat northward
contributes to the moderate wintertime climate at high latitudes, notably in
regions near England and Scandinavia.

Excessive amounts of freshwater could alter the ocean density that drives a
portion of this circulation system, diminish the amount of heat that is
transported northward, and significantly cool areas of the Northern Hemisphere.
Curry and Mauritzen report that the changes in salinity observed to date do not
appear to have changed, as yet, the ocean circulation and heat transport, but
expect continued freshening to affect the ocean conveyor in the next two
centuries.

“Precipitation and river runoff at high latitudes have been increasing,”
Curry said. “In the last decade, fresh water has been accumulating in the
Nordic Seas layer (the upper 1000 meters) that is critical to the ocean
conveyor, so it is something to watch. The Greenland ice sheet represents a wild
card,” she added. “There is an enormous amount of freshwater tied up there,
which, as it melts, will affect the headwaters of the ocean conveyor. ”

About one third of the total Atlantic MOC crosses the Greenland-Scotland
Ridge, which separates the Nordic Seas from the North Atlantic. The exchange of
waters north and south of the ridge is controlled by a contrast of ocean density
there at depths between 200 and 800 meters-the Nordic Seas being denser than the
Subpolar Basins. Adding fresh water to the ocean reduces density, removing fresh
water (or increasing its salinity) increases density. So as fresh water
accumulates in the Nordic Seas, its density will decrease, the north-south
density contrast will diminish, and the southward flows over the ridge will
weaken.

These cold, dense southward flowing waters (collectively called Nordic Seas
Overflow Waters) have been closely monitored with instrument arrays for more
than a decade, but no sustained changes have yet been measured. Of the total
19,000 cubic km of extra fresh water that has diluted the northern Atlantic
since the 1960s, only a small portion-about 4000 cubic km-remained in the Nordic
Seas; and of that amount only 2500 cubic km accumulated in the critical layer
feeding the Overflow waters. This is the reason, Curry explains, that the
overflows have not yet slowed despite all the freshening. At the rate observed
over the past 30 years, it would take about a century to accumulate enough
freshwater (roughly 9000 cubic km according to this study) in the critical
Nordic Seas layer, to significantly slow the ocean exchanges across the
Greenland-Scotland Ridge; and nearly two centuries of continued dilution to stop
them. The researchers conclude therefore that abrupt changes in ocean
circulation do not appear imminent.

Curry points out that uncertainties remain in assessing the possibility of
circulation disruptions, including future rates of greenhouse warming and
glacial melting. Most computer simulations of greenhouse warming show increased
precipitation and river runoff at high latitudes that leads to a slowing of the
Atlantic conveyor. Only one available model study, however, contains an
interactive Greenland Ice Sheet. Pooling and release of glacial meltwater,
collapse of an ice shelf followed by a surge in glacier movement, or lubrication
of the glacier’s base by increased melting are all mechanisms that could
inject large amounts of freshwater into the critical upper layers of the Nordic
Seas.

"It certainly makes sense to continue monitoring ocean, ice, and
atmospheric changes closely,” Curry said. “Given the projected 21st century
rise in greenhouse gas concentrations and increased fresh water input to the
high latitude ocean, we cannot rule out a significant slowing of the Atlantic
conveyor in the next 100 years. I emphasize that we are talking about century
timescales to witness measurable changes in the ocean transports of mass and
heat across the Greenland-Scotland Ridge-we are not suggesting that the Gulf
Stream will shut down.”

The study was funded by the National Science Foundation, a WHOI Independent
Study Award, and by the Norwegian Research Council.

# #

About Woods Hole Oceanographic Institution

Woods Hole Oceanographic Institution (WHOI) is a private, independent marine
research, engineering, and higher education organization located in Falmouth,
MA. Its primary mission is to understand the oceans and their interaction with
Earth as a whole, and to communicate a basic understanding of the ocean's role
in the changing global environment. Established in 1930 on a recommendation from
the National Academy of Sciences, the Institution is organized into five
scientific departments, interdisciplinary research institutes and a marine
policy center, and conducts a joint graduate education program with the
Massachusetts Institute of Technology.

How Much Excess Fresh Water Was
Added To
North Atlantic In Recent Decades?

SPX 17jun2005

Woods Hole MA — Large regions of the North Atlantic Ocean have been growing
fresher since the late 1960s as melting glaciers and increased precipitation,
both associated with greenhouse warming, have enhanced continental runoff into
the Arctic and sub-Arctic seas. Over the same time period, salinity records show
that large pulses of extra sea ice and fresh water from the Arctic have flowed
into the North Atlantic. But, until now, the actual amounts and rates of fresh
water accumulation have not been explicitly known.

In a paper to be published June 17 in Science, Ruth Curry of the Woods Hole
Oceanographic Institution (WHOI) and Cecilie Mauritzen of the Norwegian
Meteorological Institute quantified for the first time how much additional fresh
water caused the observed salinity changes in the northern North Atlantic Ocean,
how fast it entered the Atlantic circulation, and where that fresh water was
stored.

They report that patterns of fresh water accumulation over the past four
decades suggest that a freshening threshold important to the ocean circulation
and its poleward transport of heat could be reached in a century, although
future impacts of global warming and glacial melting make prediction imprecise
at this time.

Curry, a research specialist in the WHOI Physical Oceanography Department,
and Mauritzen, an oceanographer at the Norwegian Meteorological Institute,
analysed data collected in the North Atlantic Ocean between Labrador, Greenland
and northern Europe over the last 55 years to reconstruct the history of ocean
properties such as temperature, salinity and density.

For successive 5-year time periods, they estimated how much fresh water had
to have been added or removed to account for the top-to-bottom salinity changes
observed from 1953 to 2002.

They mapped the distribution of fresh water storage layer-by-layer to
determine where the excess was accumulating and how it was affecting ocean
density. They then used the observed rates and distribution of freshening to
estimate how long it would take to reach thresholds that would affect the
portion of ocean circulation driven by density.

In an average year, about 5000 cubic kilometers (km) of fresh water flows
from the Arctic into the North Atlantic through passages located east and west
of Greenland.

The researchers estimate that in addition to this amount, an extra 19,000
cubic km flowed into and diluted the northern seas over the 30-year time period
between 1965 and 1995.

Fully half of the excess fresh water (~10,000 cubic km) entered the system in
the late 1960s boosting the flow rate from 5000 to ~7000 cubic km per year-a 40%
increase in fresh water inflow. For comparison, the outflow from the Mississippi
River each year is about 500 cubic km, while Earth’s largest river, the
Amazon, discharges 5000-6000 cubic km annually.

Curry says the records for the 1950s and 1960s showed salinities initially
increasing in the Atlantic’s Subpolar Basins, while changes in the Nordic Seas
were relatively small. Between 1970 and 1995, however, both areas became greatly
freshened.

The team says the most striking event in their study occurred in the early
1970s and is known as the "Great Salinity Anomaly". During the late
1960s, a large pulse of fresh water entered the Nordic Seas through Fram Strait
and moved quickly southward in the East Greenland Current.

Curry says the event is well-named since it contributed about 10,000 cubic km
of extra fresh water to the northern ocean basins. (Previously, the Great
Salinity Anomaly was believed to have involved less than one quarter of that
amount.) This fresh water was eventually mixed downward in the Labrador and
Irminger Basins and then moved horizontally at mid-depths around the North
Atlantic.

Pulses of excess fresh water and ice appear to have come from the Arctic in
the 1980s and 1990s as well. Of the estimated 19,000 cubic km added to the
northern North Atlantic between 1965 and 1995, about 80% ended up in the
Subpolar Basins, which are about twice the geographic size of the Nordic Seas.

The amount of fresh water involved would be equivalent to a layer about 3
meters (roughly 9 feet) thick spread evenly over the total area of the Subpolar
Basins, and a layer about 1.8 meters (about 5 feet) thick over the Nordic Seas.

The Nordic Seas (located between Iceland, Greenland and Norway) and the
Labrador and Irminger Basins (east and west of southern Greenland) are places
where cold dense waters are formed, a critical component of the meridional
overturning circulation (MOC) and part of a great "ocean conveyor
belt" that carries warm surface waters from the tropics northward.

At high latitudes, the heat-bearing surface waters cool (the heat is released
to the overlying atmosphere) and these denser waters sink and flow southward in
the deep ocean-a process which helps keep the conveyor moving.

The transport of heat northward contributes to the moderate wintertime
climate at high latitudes, notably in regions near England and Scandinavia.

Excessive amounts of freshwater could alter the ocean density that drives a
portion of this circulation system, diminish the amount of heat that is
transported northward, and significantly cool areas of the Northern Hemisphere.

Curry and Mauritzen report that the changes in salinity observed to date do
not appear to have changed, as yet, the ocean circulation and heat transport,
but expect continued freshening to affect the ocean conveyor in the next two
centuries.

"Precipitation and river runoff at high latitudes have been
increasing," Curry said.

"In the last decade, fresh water has been accumulating in the Nordic
Seas layer (the upper 1000 meters) that is critical to the ocean conveyor, so it
is something to watch. The Greenland ice sheet represents a wild card," she
added.

"There is an enormous amount of freshwater tied up there, which, as it
melts, will affect the headwaters of the ocean conveyor. "

About one third of the total Atlantic MOC crosses the Greenland-Scotland
Ridge, which separates the Nordic Seas from the North Atlantic. The exchange of
waters north and south of the ridge is controlled by a contrast of ocean density
there at depths between 200 and 800 meters-the Nordic Seas being denser than the
Subpolar Basins.

Adding fresh water to the ocean reduces density, removing fresh water (or
increasing its salinity) increases density. So as fresh water accumulates in the
Nordic Seas, its density will decrease, the north-south density contrast will
diminish, and the southward flows over the ridge will weaken.

These cold, dense southward flowing waters (collectively called Nordic Seas
Overflow Waters) have been closely monitored with instrument arrays for more
than a decade, but no sustained changes have yet been measured.

Of the total 19,000 cubic km of extra fresh water that has diluted the
northern Atlantic since the 1960s, only a small portion-about 4000 cubic
km-remained in the Nordic Seas; and of that amount only 2500 cubic km
accumulated in the critical layer feeding the Overflow waters.

This is the reason, Curry explains, that the overflows have not yet slowed
despite all the freshening.

At the rate observed over the past 30 years, it would take about a century to
accumulate enough freshwater (roughly 9000 cubic km according to this study) in
the critical Nordic Seas layer, to significantly slow the ocean exchanges across
the Greenland-Scotland Ridge; and nearly two centuries of continued dilution to
stop them. The researchers conclude therefore that abrupt changes in ocean
circulation do not appear imminent.

Curry points out that uncertainties remain in assessing the possibility of
circulation disruptions, including future rates of greenhouse warming and
glacial melting.

Most computer simulations of greenhouse warming show increased precipitation
and river runoff at high latitudes that leads to a slowing of the Atlantic
conveyor. Only one available model study, however, contains an interactive
Greenland Ice Sheet.

Pooling and release of glacial meltwater, collapse of an ice shelf followed
by a surge in glacier movement, or lubrication of the glacier’s base by
increased melting are all mechanisms that could inject large amounts of
freshwater into the critical upper layers of the Nordic Seas.

"Given the projected 21st century rise in greenhouse gas concentrations
and increased fresh water input to the high latitude ocean, we cannot rule out a
significant slowing of the Atlantic conveyor in the next 100 years. I emphasize
that we are talking about century timescales to witness measurable changes in
the ocean transports of mass and heat across the Greenland-Scotland Ridge-we are
not suggesting that the Gulf Stream will shut down."